With the increasing use of full-color, computer-generated and photographic transparencies, and liquid crystal display (LCD) projection panels, there is a need for projection systems of increased brightness. This has been recently addressed by the use of higher wattage tungsten-halogen lamps, metal-halide lamp technology, and high-efficiency anti-reflection coatings on the optical components. The use of higher wattage tungsten filament lamps increases the difficulty of cooling, and arc discharge lamps, such as metal halide, are relatively expensive.
In the past, several attempts to increase the illumination level of projection systems were characterized by the use of multiple lower wattage lamps. In the case of episcopic projection, commonly practiced in opaque projectors, it is simply necessary to illuminate the opaque copy with light from several sources. The scattered light from the copy which enters a projection lens is then directed to the screen. This type of projection system is described, for example, in U.S. Pat. No. 4,979,813.
In diascopic projection, light from a lamp is collected by single or multiple condensing lenses, passes through a projection transparency, and is focused to the projection lens. This mode of projection is commonly used in 35 millimeter and overhead projectors, and gives a brighter projected image than episcopic projection. See, for example, U.S. Pat. Nos. 3,547,530 and 3,979,160. However, since the condensing lens system can only efficiently focus light from one single point (lamp position) to another single point (projection lens position), diascopic projection is usually limited to the use of a single lamp. This inherently limits the brightness which may be achieved on the screen.
Several attempts have been made to efficiently combine the output of multiple lamps and bring them to a common focus. U.S. Pat. No. 1,887,650 describes a system combining eight lamps, U.S. Pat. No. 3,770,344 describes an overhead projector combining four lamps, and Japanese patent nos. 4-179046 and 5-199485 describe projectors combining two lamps. All of these devices suffer from a common deficiency in that the combined output from the lamps is not truly integrated. The combined lamp output beams are contiguous, yet spatially separated. The result is that should one lamp fail, the screen image brightness does not diminish uniformly, but rather individual sections of the screen image become completely dark, making part of the screen image unreadable.
True integration of the light from multiple sources, allowing them to be focused to a common point, is a more demanding task. U.S. Pat. No. 4,952,053 describes an overhead projector combining two lamps, U.S. Pat. No. 5,231,433 describes a method for integrating two collimated light beams by means of a linear grooved reflector or a linear grooved refracting element. Japanese patent no. 5-232399 describes a method for combining and integrating the output of two lamps by means of a beam splitter and multiple-pass reflections. The efficiency of these systems is limited by the achievable reflectance of the reflector coatings, geometric shading losses, and by the high chromatic dispersion of the refracting elements.